Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the plate is exposed with an ultraviolet light from a lamp through a separate photomask film having predetermined imaging pattern that is placed between the light source and the plate, and the exposed plate is developed with a liquid developer to bare the substrate in the non-hardened or solubilized areas. Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser wavelength, allowing the elimination of the photomask film.
On-press developable lithographic plates have been disclosed in the literature. Such plates can be directly mounted on press after imagewise exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed, allowing the reduction of labor and the elimination of hazardous waste. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,482,571.
Laser sensitive on-press developable lithographic plates generally comprise on a substrate a photosensitive layer. A water-soluble overcoat is usually further coated on the photosensitive layer to achieve fast photospeed as required for laser imaging. Such an overcoat is typically removed on press with fountain solution during on-press development. Although removing the overcoat by rinsing with water has been suggested in the patent literature, such water rinse introduces additional liquid waste.
While laser sensitive on-press developable lithographic plates having a water-soluble overcoat allow fast photospeed, it has the drawbacks of contamination to the fountain solution and/or limited white room light stability. It would be desirable if a laser sensitive on-press developable lithographic plate and/or method of using it can be designed so that the plate can have fast photospeed during laser exposure, does not cause contamination to the fountain solution, can be handled freely or more freely under white room light during on-press development, and does not generate liquid waste.
The inventor has found that such a desire can be achieved by a method of mechanically peeling off the overcoat of a lithographic plate after laser exposure and before on-press development.